Adaptive cruise control (hereinafter referred to as “ACC”) and collision mitigation brake systems (for example, a pre-crash safety system (PCS: registered trademark)) are known which control acceleration and deceleration of a vehicle to avoid a collision with a preceding vehicle. In ACC, acceleration and deceleration of a vehicle are controlled by adjusting the distance between the vehicle and a preceding vehicle based on the traveling speed of the vehicle so as to avoid a collision with the preceding vehicle (see, for example, Patent Literature 1). A collision mitigation brake system compares the distance between a vehicle and an obstacle, for example, a preceding vehicle with the braking distance of the vehicle and warns the driver of the vehicle or, by generating a braking force, controls acceleration and deceleration of the vehicle so as to avoid a collision between the vehicle and the obstacle.